Photovoltaic component and module

ABSTRACT

A photovoltaic component with at least one tricrystalline wafer ( 2 ) that has a certain basic doping, a light-receiving side ( 3 ), an electric bonding side ( 4 ) opposite the light-receiving side ( 3 ), and at least one interdigital semiconductor structure ( 5 ) arranged on the electric bonding site ( 4 ) with at least on n-type semiconductor part-structure ( 6 ) and at least one p-type semiconductor part-structure ( 7 ) arranged at a certain interval ( 8 ) to the n-type semiconductor part-structure ( 6 ). One of the semiconductor part-structure and the silicon wafer ( 2 ) thus form a p-n junction ( 9 ).

[0001] The invention relates to a method of manufacturing a photovoltaiccomponent with a silicon wafer and to a photovoltaic module with atleast one silicon wafer. The silicon wafer has a predetermined basicdoping, a light-receiving side, an electric bonding side facing awayfrom the light-receiving side, and at least one interdigitalsemiconductor structure arranged on the electric bonding side with atleast one n-type semiconductor part-structure and at least one p-typesemiconductor part-structure arranged at a predetermined distance fromthe n-type semiconductor part-structure. One of the semiconductorpart-structures forms a p-n junction with the silicon wafer.

[0002] An example of a photovoltaic component is disclosed inInternational patent application publication No. 98/25 312. Thispublication relates to a conventional photovoltaic component, wherein ap-n junction is formed at the light-receiving side of a p-doped siliconwafer. It is proposed to make an electric contact between thelight-receiving side at the backside of the wafer. To this end, thesilicon wafer is cut into strips. Each strip is first provided with ann-doped emitter layer that covers all of its surfaces and on top of thatlayer a passivating layer is arranged. Then a first set of contacts ismade on the backsides of the strips, which contacts are p-doped andextend through the passivating layer and the emitter layer into thesilicon wafer, and a second set of contacts is made, that are arrangedin the slits and extend through the passivating layer into the emitterlayer so as to form an electric contact with that emitter layer. Theemitter layer and the second set of contacts provide an electricconnection between the front side of the silicon wafer to its backside.

[0003] Another photovoltaic component is disclosed in German patentapplication publication No. 195 25 720.

[0004] Manufacturing the known photovoltaic component comprises thesteps of

[0005] (a) providing a silicon wafer having a light-receiving side andan electric bonding side opposite to the light-receiving side with apredetermined basic doping in the form of a p-type doping;

[0006] (b) providing the silicon wafer with a passivating layer at theelectric bonding side;

[0007] (c) making a plurality of grooves arranged one next to the otherin the passivating layer at the electric bonding side, which grooves canextend into the silicon wafer;

[0008] (d) making at the grooves n-type doping areas in the siliconwafer to obtain n-type semiconductor part-structure;

[0009] (e) making a plurality of p-type thick-film electric contacts onthe passivating layer, between adjacent grooves to obtain p-typesemiconductor part-structure and a contacting part-structure; and

[0010] (f) making in each of the grooves a thin film contact to obtainan auxiliary contacting part-structure.

[0011] The photovoltaic component is a solar cell that serves to convertelectromagnetic energy into electrical energy. The centrepiece of thecomponent is a silicon wafer. The silicon wafer is a flat,mono-crystalline body of silicon with two main surfaces directedopposite to each other. The thickness of the silicon wafer is normallyabout 160 μm (micrometer) or more, which is substantially smaller than alateral extension (diameter) of the silicon wafer. The lateral extensionof the silicon wafer for example is about 10 cm.

[0012] The silicon wafer has silicon as its semiconductor base material.Additionally the silicon wafer has a predetermined basic doping. Apredetermined basic doping is understood to mean the introduction of apredetermined dopant or impurity atom into a base material for changingone electrical property of the base material. Doping can result in twotypes of conductivity, the p-type conductivity and the n-typeconductivity. A suitable dopant for silicon is an element of the thirdmain group of the periodic table for producing p-type conductivity andan element of the fifth main group of the periodic table for producingn-type conductivity.

[0013] The silicon wafer has a light-receiving side (front side), whichcan be provided with a texture. When the photovoltaic component is inoperation, electromagnetic radiation reaches into the silicon wafer viathe light-receiving side. For supplying electric energy to an externalcircuit an interdigital semiconductor structure is arranged on theelectric bonding side (rear side) of the silicon wafer. Thissemiconductor structure consists of an n-type semiconductorpart-structure and a p-type semiconductor part-structure. One of thesemiconductor part-structures together with the silicon wafer forms thep-n junctions, which are necessary for the production of thephotocurrent.

[0014] The semiconductor part-structures are arranged at a predetermineddistance to each other. This means that there are no p-n junctionsbetween the interdigital semiconductor part-structures. Thesemiconductor part-structures are for example grid-like or finger-like.They consist of a plurality of webs, wherein one web of a semiconductorpart-structure is separated from a web of the other semiconductorpart-structure for example by a groove or an interval. The semiconductorpart-structures engage with each other in such a way that one surface isas large as possible, which is produced by the p- and n-conductingsemiconductor part-structures arranged opposite each other.

[0015] One of the semiconductor part-structures together with thesilicon wafer forms a base of the photovoltaic component. The othersemiconductor part-structures forms an emitter of the photovoltaiccomponent. For electrically contacting the interdigital semiconductorstructure an interdigital contacting structure is mounted on theinterdigital semiconductor structure with a semiconductor part-structurefor electrically contacting the n-type semiconductor part-structure, anda contacting part-structure for electrically contacting the p-typesemiconductor part-structure.

[0016] It is an object of the present invention to provide a simplemethod of producing an electric bonding side of a photovoltaic componentbased on a silicon wafer.

[0017] To this end the present invention provides a method ofmanufacturing a photovoltaic component, which method comprises providinga silicon wafer having a light-receiving side and an electric bondingside opposite to the light-receiving side with a predetermined basicdoping, characterized in that the method further comprises the steps of:

[0018] (a) producing a doped layer on the electric bonding side of thesilicon wafer only, wherein the doping of the doped layer differs fromthe predetermined doping of the silicon wafer to get a p-n junction inthe silicon wafer;

[0019] (b) applying on the doped layer an n-type dopant for producing ann-type semiconductor part-structure and a p-type dopant for producing ap-type semiconductor part-structure of an interdigital semiconductorpart-structure, and allowing the dopants to diffuse into the dopedlayer, thereby forming the semiconductor part-structures; and

[0020] (c) separating the semiconductor part-structures by removing thedoped layer in the intervals between the semiconductor part-structures.

[0021] An advantage of the method according to the present invention isthat no grooves have to be made in the passivating layer, and thereforethe method according to the present invention is more simple andeconomical. The layer thickness of the silicon wafer is limited to 160μm or more. This is explained by the fact that the photovoltaiccomponent of the type in question is equipped with a mono-crystallinesilicon wafer. A layer thickness below 160 μm does not guaranteemechanical stability of the mono-crystalline silicon wafer. In order tohave a silicon wafer that has a smaller thickness, the silicon wafer issuitably a tri-crystalline silicon wafer, as described in German patentapplication publication No. 43 43 296.

[0022] According to the method of the invention a so-called reverse sidebonding of the silicon wafer or the photovoltaic component takes place.In this way the shading of a photovoltaic component can be preventedwith a silicon wafer and at the same time mechanical stability of thesilicon wafer can be guaranteed at a layer thickness of below 160 μm.

[0023] The predetermined basic doping of the silicon wafer is either ann- or p-type basic doping, wherein the n-type doping is dominated bynegative charges (electrons) and the p-type is dominated by positivecharges (holes). Accordingly either the n-type semiconductorpart-structure or the p-type semiconductor part-structure forms with thesilicon wafer the p-n junction. This means that the silicon wafertogether with one of the semiconductor part-structures forms the base ofthe photovoltaic component. The emitter of the photovoltaic component isformed by the other semiconductor part-structure.

[0024] The absorption of a photon by the photoactive material of thesilicon wafer causes a charge separation to take place. The dopingcauses a certain minority charge carrier to be formed. In the case of ap-type doping the minority charge carrier is an electron, in the case ofan n-type doping the minority charge carrier is a “positively charged”electron hole. The doped layer has a doping that is directed opposite tothe predetermined basic doping of the silicon wafer. This means that inthe case of an n-type basic doping of the silicon wafer, the layerdoping of the doped layer is p-type and vice versa.

[0025] According to further embodiment the light-receiving side and/orthe electric bonding side of the silicon wafer are provided with apassivation layer. The passivation layer is directly applied on asurface of the silicon wafer and reduces primarily the possibility thata surface-charge-recombination occurs which reduces a photocurrent. Thepassivation layer on the electric bonding side additionally takes overthe task of an electrical insulation of the p- and n-type semiconductorpart-structures against each other. The passivation layer is located inan interval between the p- and n-type semiconductor part-structures. Thepassivation layer contains in particular a passivating agent that isselected from the group silicon oxide and/or silicon nitride.

[0026] In a further embodiment, at least one encapsulation layer isapplied on the passivation layer, wherein the encapsulation layercomprises at least one transparent material. Especially an encapsulationon the passivation layer on the light-receiving side consists of thetransparent material. The transparent material ensures that theelectromagnetic radiation can penetrate the photo-electrically activesilicon wafer. Preferably the transparent material is photostable. Thismeans that a transmission of the encapsulation from the transparentmaterial remains essentially constant during the operation of thephotovoltaic component. The transmission for a certain wavelength of theelectromagnetic radiation will not be reduced for example by aphoto-induced process. The encapsulation is transparent preferably inthe region between 300 nm (nanometer) and 1200 nm. In this region thesilicon absorbs electromagnetic radiation necessary for the chargeseparation. In particular the transparent material is selected from thegroup glass and/or ethyl-vinyl-acetate (EVA). Apart from this it ispossible to use any other synthetic material that is photostable underthe conditions of use of the photovoltaic component. The conditions ofuse relate for example to one wavelength region and one intensity of theelectromagnetic radiation.

[0027] The invention further relates to a photovoltaic module having alight-receiving side and an electric bonding side opposite thelight-receiving side, which module comprises at least one silicon waferthat has a predetermined basic doping, which at least one silicon waferhas a light-receiving side and an electric bonding side opposite thelight-receiving side, which wafers are so arranged that thelight-receiving sides of the wafers form the light-receiving side of thephotovoltaic module and the electric bonding sides form the electricbonding side of the photovoltaic component, and electric connectionsbetween the silicon wafer at the bonding side, characterized in that thewafer is a tri-crystalline silicon wafer having a predetermined basicdoping, and in that each wafer is provided at the bonding side with atleast one interdigital semiconductor structure comprising at least onen-type semiconductor part-structure provided with a fist contactingpart-structure and at least one p-type semiconductor structure providedwith a second contacting part-structure, wherein either the n-typesemiconductor part-structure or the p-type semiconductor part-structureforms with the wafer a p-n junction.

[0028] Each silicon wafer itself forms a photovoltaic component in theform solar cell. The chain of silicon wafers together forms a solarmodule. The special advantage of such an arrangement consists in thefact that an electrical contact to the p-type semiconductorpart-structure and to the n-type semiconductor part-structure isarranged on one side of the silicon wafer and thus can be easilymanufactured by automation. Tape lapping, interconnecting, laminatingand encapsulating the solar cells or solar modules significantlyincreases the yield compared to the electric bonding of the solar cellson both sides. Obsolete is a complicated interconnection of severalsolar cells into a solar module, in which the light-receiving sides andthe bonding sides of the solar cells must be connected tension-free by afolded or curved metal contact strip.

[0029] The invention will now be described by way of example in moredetail with reference to the accompanying drawings, wherein

[0030]FIGS. 1a and 1 b each show schematically a photovoltaic componentin cross-section;

[0031]FIG. 2 shows schematically a cut-out of a photovoltaic componentin cross-section;

[0032]FIGS. 3a and 3 b each show schematically an interdigital bondingstructure in plan view; and

[0033]FIGS. 4a and 4 b show schematically various process stages formanufacturing the photovoltaic components.

[0034] The photovoltaic component 1 is a solar cell 101 and consists ofa silicon wafer 2 (FIG. 1a). The silicon wafer 2 has boron as its dopantand thus the predetermined basic doping is a p-type doping.

[0035] The silicon wafer 2 has a layer thickness 22 of approximately 100μm. The silicon wafer has a light-receiving side 3. During operation ofthe photovoltaic component 1 electromagnetic radiation 19 enters thephoto-electrically active silicon wafer 2. On the light-receiving side 3of the silicon wafer 2 is arranged a passivation layer 10 of siliconoxide. An encapsulation 11 is arranged over the passivating layer 10.The encapsulation 11 is an anti-reflex layer for increasing lightcoupling into the silicon wafer 2. The silicon wafer 2 further comprisesan electric bonding side 4 that is at the side of the silicon wafer 2opposite to the light-receiving side 3. On the electric bonding side 4is arranged an interdigital semiconductor structure 5. The semiconductorstructure 5 consists of an n-type semiconductor part-structure 6 and ap-type semiconductor part-structure 7.

[0036] These semiconductor part-structures are arranged such that thereis an intervening space or interval 8 between them (see FIG. 2). Then-type semiconductor part-structure 6 and p-type semiconductorpart-structure 7 do not have a common p-n junction. In contrast to this,the n-type semiconductor part-structure 6 and the p-type doped siliconwafer 2 form a p-n junction 9.

[0037] The electric bonding side 4 also has a passivation layer 10. Thepassivation layer 10 is arranged on a surface of the electric bondingside 4 between the interdigital semiconductor structures 6 and 7. Thepassivation layer 10 not only serves to reduce the photocurrent reducingsurface-charge-recombination of the separated charge carriers. Thepassivation layer 10 also serves to electrically insulate the n-typesemiconductor part-structure 6 and the p-type semiconductorpart-structure 7 from each other.

[0038] In order to allow an electric contact with the photovoltaiccomponent 1, the interdigital semiconductor structure 5 is provided withan interdigital contacting structure 16 comprising a first contactingpart-structure 17 for an electric connection with the n-typesemiconductor part-structure 6 and a second contacting part-structure 18for an electric connection with the p-type semiconductor part-structure7. The web width of the interdigital contacting structure 16 correspondsessentially the web width of the interdigital semiconductor structure 5.

[0039] A first possible interdigital contacting structure 16 andtherefore also an interdigital semiconductor structure 5 is indicated inFIG. 3a. First and second contacting part-structures 17 and 18 andtherefore the n-type semiconductor and p-type part-structures 6 and 7thus form a comb-like or finger-like structure. These part-structuresinterengage without touching each other. An alternative embodiment tothe interdigital contacting structure 16 and the interdigitalsemiconductor structure 5 is reproduced in FIG. 3b.

[0040] The invention further provides a photovoltaic component 1 in theform of a photovoltaic module 102, see FIG. 1b. The photovoltaic module102 comprises a plurality of silicon wafers 2 (solar cells 101)described above. The silicon wafers 2 of the solar module 102 arearranged to form a chain 12 in such a way that light-receiving sides 3of the silicon wafers 2 form a common light-receiving side 13 of thephotovoltaic module 102, and the electric bonding sides 4 of the siliconwafers 2 form a common bonding side 14 of the photovoltaic module 102.The silicon wafers 2 are arranged side by side equally oriented. Thechain 12 of the silicon wafers 2 is a solar module 102 which is formedof an easily to manufacture electrical serial interconnection 15 of theindividual silicon wafers 2 or the individual solar cells 101. The solarmodule 102 as a whole is embedded in an encapsulation 111 ofethyl-vinyl-acetate. The encapsulation 111 together with the solarmodule 102 is embedded in a second encapsulation 112 of glass. In analternative embodiment to this the outer encapsulation consists of aglass of different transparent material.

[0041] Reference is now made to FIGS. 4a and 4 b showing several stagesof the method according to the present invention.

[0042] In the method of the present invention at first a silicon wafer 2having a light-receiving side and an electric bonding side opposite tothe light-receiving side with a predetermined basic doping is provided.To this end a silicon wafer is sawn from crystal of p-type doped silicon(boron doping). The silicon wafer is cleaned by a conventional methodand any surface unevennesses removed by etching. During sawing it ispossible that a micro-crack appears on the surface of the silicon waferthat can be removed by the etching process. This causes the mechanicalstability of the silicon wafer to be increased.

[0043] Liquid phosphorous-containing composition is flung onto theelectric bonding side 4 of the silicon wafer 2 to be manufactured (workstage 401). A suitable composition is phosphorous-Siodop B-430, fromMerck, which contains phosphorous ions and a mixture of tetraethylsilicate, ethanol, ethyl acetate and acetic acid (phosphorous-Siodop isa trademark). Applying the composition results in a layer composite 23of silicon wafer and applied thereon phosphorous-containing layer 24.The phosphorous-layer 24 is a few μm thick. The layer composite 23 isheated at approximately 200° C. for 10 min, which causes thephosphorous-containing layer to harden (work stage 402).

[0044] The phosphorous is then diffused from the hardenedphosphorous-containing layer into the p-type silicon wafer 2 (work stage403). The production of the doped layer 25 with n-type doping takesplace. This is achieved at a temperature of approximately 850° C. whilsttempering for 30 min. The resulting doped layer 25 leads to the p-njunction 9 necessary for the operation of the photo-voltaic component 1.The doped layer 25 is a few tenths μm thick.

[0045] During hardening of the phosphorous-containing layer or duringthe diffusion of the phosphorous, a phosphorous-glass-layer 26 isformed. This phosphorous-glass-layer 26, after the production of thedoped layer 25, is etched with aqueous hydrofluoric acid (work stage(404).

[0046] The interdigital semiconductor structure 5 is now produced on orin the doped layer 25. For this p- and n-type dopants are applied to thedoped layer 25. (work stage 405). This is achieved with the aid of ascreen printing method in which an n-paste 30 containing the n-typedopant 27 and a p-paste 31 containing the p-type dopant 28 are printedin a form which corresponds to the interdigital semiconductorpart-structures 6 and 7 to be produced (web width 29 between 40 and 100μm). The pastes 30 and 31 function as the sources of the n- and p-typedopants 27 and 28.

[0047] The dopants 27 and 28 are burnt-in in a continuous furnace (workstage 406). During burning-in the dopant (aluminium) contained in thep-paste 31 is driven through (fired through) the n-type doped layer 25.During this process beneath the p-paste 31 the n-doping of the dopedlayer 25 is overcompensated. This results in the p-type semiconductorpart-structure 7 causing an electrical contact to be made with thep-type silicon wafer 2. The n-type semiconductor part-structure 6 isproduced beneath the n-paste 30 during burning-in. During burning-in ofthe n-paste 30 an n-type character of the doped layer 25 remains.

[0048] Furthermore, during burning-in the interdigital contactingstructure 16 is formed. From the p-paste 31 is produced the secondcontacting part-structure 18 for the electric contact of the p-typesemiconductor part-structure 7 and from the n-paste 30 the firstcontacting part-structure 17 for electrically bonding the n-typesemiconductor part-structure 6.

[0049] For electrical insulation between p-type semiconductorpart-structure 6 and n-type semiconductor part-structure 7 there takesplace a plasma etching of the electric bonding side 4, wherein theinterdigital contacting structure 16 is used as the etching mask (workstage 407). The free spaces of the doped layer 25, which are not coveredby the contacting part-structures 17 and 18, between the n- and p-typesemiconductor part-structures 6 and 7 are thus etched down to thesilicon wafer 2. This results in etching grooves by which the n- andp-type semiconductor part-structures are separated from each other.

[0050] The p-type semiconductor part-structure 7 and the silicon wafer 2together form the basis 32 of the photovoltaic component 1. The n-typesemiconductor part-structure 6 forms the emitter 33 of the photovoltaiccomponent 1.

[0051] To prevent the surface-charge-recombination and for theadditional insulation of the n- and p-type semiconductor part-structures6 and 7 a passivation layer 10 of silicon oxide is formed on the surfaceof the silicon wafer 2 between the n- and p-type semiconductorpart-structures 6 and 7 (work stage 408).

[0052] Suitably the silicon wafer is a tri-crystalline wafer, which issawn from a tri-crystalline crystal of p-type doped silicon (borondoping).

[0053] In the above described example, the silicon wafer 2 was providedwith a p-type doping, and the doped layer is of the n-type. However, inan alternative embodiment of the invention, the silicon wafer isprovided with a n-type doping, and the doped layer is of the p-type. Asuitable dopant for silicon is an element of the third main group of theperiodic table for producing p-type doping and an element of the fifthmain group of the periodic table for producing n-type doping.

[0054] The method according to the present invention allows easymanufacture of the component by automated means and it is therefore aneconomically attractive method. This also concerns the interconnectionof the component in the form of a solar module. An efficiency rating ofthe component of 20% is thus possible.

1. Method of manufacturing a photovoltaic component, which methodcomprises providing a silicon wafer having a light-receiving side and anelectric bonding side opposite to the light-receiving side with apredetermined basic doping, characterized in that the method furthercomprises the steps of: (a) producing a doped layer on the electricbonding side of the silicon wafer, wherein the doping of the doped layerdiffers from the predetermined doping of the silicon wafer to get a p-njunction in the silicon wafer; (b) applying on the doped layer an n-typedopant for producing the n-type semiconductor part-structure and ap-type dopant for producing the p-type semiconductor part-structure in afrom that corresponds to interdigital semiconductor part-structures, andallowing the dopants to diffuse into the doped layer, thereby formingthe semiconductor part-structures; and (c) separating the semiconductorpart-structures by removing the doped layer in the intervals between thesemiconductor part-structures.
 2. Method according to claim 1, whereinthe silicon wafer is a tri-crystalline silicon wafer.
 3. Methodaccording to claim 1 or 2, wherein step (b) comprises applying on thedoped layer a metal paste containing an n-type dopant and a metal pastecontaining a p-type dopant on the doped layer in a from that correspondsto the interdigital semiconductor part-structures, and burning-in thedopants to form the semiconductor part-structures and an interdigitalcontacting structure.
 4. Method according to any one of the claims 1-3,wherein for the basic doping of the silicon wafer p-doping is used andfor producing the doped layer phosphorus is used as the dopant. 5.Method according to claim 4, wherein a liquid phosphorous-containingsolution is applied on the silicon wafer to obtain a composite that isheated.
 6. Method according to any one of the claims 1-5, furthercomprising applying a passivation layer on the light-receiving sideand/or on the bonding electric bonding side of the silicon wafer. 7.Method according to claim 6, wherein applying the passivation layerincludes applying a layer of silicon dioxide or of silicon nitride. 8.Method according to claim 6 or 7, further comprising applying at leastone encapsulation layer on the passivation layer, wherein theencapsulation layer comprises at least one transparent material. 9.Photovoltaic module having a light-receiving side and an electricbonding side opposite the light-receiving side, which module comprisesat least one silicon wafer that has a predetermined basic doping, whichat least one silicon wafer has a light-receiving side and an electricbonding side opposite the light-receiving side, which wafers are soarranged that the light-receiving sides of the wafers form thelight-receiving side of the photo-voltaic module and the electricbonding sides form the electric bonding side of the photovoltaiccomponent, and electric connections between the silicon wafer at thebonding side, characterized in that the wafer is a tri-crystallinesilicon wafer having a predetermined basic doping, and in that eachwafer is provided at the bonding side with at least one interdigitalsemiconductor structure comprising at least one n-type semiconductorpart-structure provided with a fist contacting part-structure and atleast one p-type semiconductor structure provided with a secondcontacting part-structure, wherein either the n-type semiconductorpart-structure or the p-type semiconductor part-structure forms with thewafer a p-n junction.